In a precombustion-chamber type lean premixed gas internal combustion engine (hereinafter, referred to in short as a gas engine), there is a starting method adopting slow air turning, in which compressed air (not comprising fuel gas) is introduced into cylinders to rotate a gas engine before starting an air starting device, for the purpose of checking the inside of the cylinders, securing lubrication of the sliding surfaces of the cylinders and pistons that slide inside the cylinders, and reducing the preparation work before starting. In a gas internal combustion engine that performs slow air turning, compressed air that is used during slow air turning and until startup of the engine using compressed air (not comprising fuel gas) is performed subsequently to slow air turning performs the role of exhaust purge before startup of the gas engine.
However, to secure supply and safety of the fuel gas for the gas engine, a variety of devices, valves, and the like, are provided, from which unpredictable leakage of the fuel gas may occur. Fuel gas having leaked from the above devices, valves, and the like may flow to and accumulate in the gas engine, an exhaust channel, and the like. If the gas engine is switched to fuel-gas ignition startup under such a condition, the accumulating fuel gas undergoes abnormal combustion and damages the gas engine, the exhaust channel, and the like.
Patent Document 1 discloses performing a dry-air injecting block of injecting dry air into an intake channel and a cranking block of igniting and combusting air-fuel mixture with a spark plug and starting rotation of an output shaft of a gas engine after the dry-air injecting block. As a result, if the gas engine is started, dry air is injected into an air-supply channel before operation of the spark plug, which reduces the absolute humidity inside the intake channel. Thus, if the spark plug operates for the first time after startup, air with low humidity is supplied into a combustion chamber. Accordingly, the surface of electrodes of the spark plug is less likely to get moist, which makes it possible to maintain good insulation resistance between the electrodes, so that the spark plug generates sparks normally and misfire is prevented.
Further, in Patent Document 1, pre-purge, which is to discharge remaining gas in a combustion chamber to the outside via the exhaust channel, is performed. At this time, the first injector, the second injector, and the spark plug are maintained to be shut down. An injection switch valve closes and a starter inlet valve opens. As a result, a starter motor starts to operate and a flywheel is driven to rotate together with an output shaft. Then, the pistons reciprocate without the air-fuel mixture undergoing ignition combustion. In this way, intake valves and exhaust valves are driven by the output shaft, performing opening/closing motion in accordance with the stroke of the pistons. As the reciprocating motion of the pistons is repeated, the remaining gas in the combustion chamber is forced out to the exhaust channel. Instead, dry air is supplied to a main combustion chamber and a precombustion chamber from the intake channel. In this way, it is possible to ventilate the inside of the main combustion chamber and the precombustion chamber with dry air, which makes it possible to prevent non-combusted gas or soot from being incorporated into air-fuel mixture if the air-fuel mixture undergoes ignition combustion for the first time upon startup. The reciprocating motion of the pistons is repeated a certain number of times that is sufficiently large to discharge gas from the main combustion chamber and the precombustion chamber and that is sufficiently small so that air in the supply-air channel does not completely pass through the main combustion chamber.